The Origin and Distribution of the Chest-Nut-Backed Chickadee

THE Chestnut-backed Chickadee (Warus rufescens) is a boreal species of peculiarly limited distribution.It is almost exclusively confined to the humid Pacific Coast region of North America, within which it is the most abundant, and in many places the only, member of the genus ]'arus present.We find it characteristically at home within the densest coniferous forests, or along their edges, where there is much shade and an even temperature.The range of the Chestnut-backed Chickadee is nearly two thousand miles long north and south, extending from a little north of Sitka, Alaska, to some forty miles below Monterey, California.(See Map I.) But its width is very narrow, only within the confines of Oregon and Washington exceeding one hundred miles and elsewhere usually much less, save for one or two isolated interior colonies to be mentioned later.The influences determining this queer-shaped distribution area may be safely assumed to be atmospheric humidity, with associated floral conditions.For this habitat coincides quite accurately with the narrow coastal belt of excessive cloudy weather and rainfall. The specific character distinguishing ]arus rufescens from allother American chickadees is the color of the back, which is an intense rusty brown approaching chestnut.It is of common note that the most evident effects of similar climatic conditions on other animals is a corresponding intensification of browns, especially dorsally.We may therefore consider the Chestnut-backed Chickadee, as indicated by its chief specific character, to be a product exclusively of the peculiar isohumic area to which we find it confined.Warus rzscens, from Sitka to Monterey, has a chestnut-colored back.And from Sitka to Point Arena, between which we find the extremest humidity, another conspicuous character is uniform,the color of the sides, which are also deep rusty brown.But from Point Arena south to San Francisco Bay (Marin District), these GRINNELL, Chestnut-backed Chickadee.